This invention relates to a means for transporting a copy sheet from the transfer station to the fuser station of a copying machine. More particularly, it relates to a short paper path configuration within a copier in which copy paper is simultaneously subjected to both transfer and fusing of an image.
In a transfer electrostatographic process such as conventional transfer xerography, in which an image pattern of dry particulate unfused toner material is transfered to a final image support surface, e.g., a copy sheet from an initial image bearing surface, e.g., a charged photoreceptor surface developed with toner, the transferred toner is typically only loosely adhered to the final support surface after transfer, and is easily disturbed by the process of stripping the final support surface away from the initial support surface and by the process of transporting the final support surface to the toner fusing station. The final support surface preferably passes through a fusing station as soon as possible after transfer so as to permanently fuse the toner image to the final support surface, thereby preventing smearing or disturbance of the toner image by mechanical agitation or electrical fields. For this reason, and also for reasons of simplifying and shortening the paper path of the copier and space savings, it is desirable to maintain the fusing station as close as possible to the transfer station. A particularly desirable fusing station is a roll type fuser, wherein the copy sheet is passed through a pressure nip between two rollers, preferably at least one of which is heated and at least one of which is resilient.
However, when such a fuser roll nip for the final support surface is located close enough to the transfer station so that a lead portion of the final support surface can be in the fuser roll nip simultaneously with the rear or trailing portion of that same final support surface still being in contact with the photoreceptor, then a serious problem can arise, to which the present invention provides a solution. This problem is that of smears or skips in the unfused toner image which has been, or is being, transferred to the trailing portion of the final support surface. This condition is caused by relative movement or slippage between the initial support surface and the final support surface in those areas where they are still in contact, i.e., those areas of the final support surface which have not yet been stripped away from the initial support surface. A source of such slippage is a speed mismatch between the nip speed of the fuser rolls (the speed at which the fuser is pulling the lead edge of the paper through the fuser) relative to the surface speed of the initial support surface. If the fuser roll nip speed is slower, the final support can slip backwards relative to the initial image support surface. If the fuser roll is faster, the final support material can be pulled forward relative to the image on the initial support surface. In either case this can cause the aforementioned smears or skips in the toner image being transferred to the trailing edge of the final support, or image elongation.
An exactly equal velocity drive connection between the initial support surface and the fuser rolls is difficult to maintain. Also, there is a further complication that the actual sheet driving velocity of the fuser roll nip can change with changes in the effective diameter of the driving roll in the nip. This can occur from replacement of the rollers, or changes in the resilient deformation of the rollers due to changes in the applied nip pressure, materials aging, temperature effects, etc. In addition, paper of different thicknesses travel through a fuser at different speeds. Thus, equal speed is difficult to maintain between the fuser roll nip and the photorecepter surface in a commercial apparatus and may require increased maintenance and speed adjustment mechanisms.
Where the spacing between the fusing station and the transfer station is greater than the dimensions of the copy sheet, and a separate two-speed sheet transport is provided therebetween, then substantially different fuser roll nip speeds can be provided, as in U.S. Pat. No. 3,794,417, issued Feb. 26, 1974, to J. A. Machmer. However, this has the noted disadvantages of requiring additional space, increased unfused image sheet handling, and also the additional complexity and expense of the additional transport mechanism.
It is known in the electrostatographic copying art to form a buckle in a copy sheet in its movement through the copier at other locations and for other functions. For example, it is known to interrupt the forward movement of a copy sheet with registration fingers and to form a buckle in the copy sheet by its continued feeding by upstream feed rollers to provide registration of the lead edge of the copy sheet before the copy sheet is fed into the image transfer station, e.g., U.S. Pat. No. 3,601,392, issued Aug. 24, 1971, to Merton R. Spear, Jr., et al. It is also known to provide for pre-form a buckle in a web of copy material to compensate for the braking of the web during a cutting operation in which the web is cut into individual sheets, e.g., U.S. Pat. No. 3,882,744, issued May 13, 1975, to Alan F. McCarroll. The later patent also illustrates that the copy web may be pre-formed into an initial convex buckle over an apertured surface and that air pressure may be utilized to expand the buckle when the web is stopped downstream thereof.
U.S. Pat. No. 3,774,907, issued Nov. 27, 1973, to Stephen Borostyan illustrates a vacuum sheet stripping device for removing copy sheets from the initial image support member and advancing them to a roll fuser, wherein the copy sheets assume a convex shape. A rotating cylindrical apertured vacuum member is utilized, to which the copy sheet is attracted. During a portion of its rotation, the vacuum is automatically cutoff to the vacuum stripping member to release the copy sheet.
U.S. Pat. No. 3,508,824, issued Apr. 18, 1970, to R. K. Leinback et al. describes a conductive curved guide plate for attracting a copy sheet at the stripping area and guiding it towards a fusing station.
The present invention provides a speed mismatch compensation system which allows the fusing roll nip to be closely spaced from the transfer station of a printer or copier, by a distance less than the movement dimension of an individual copy sheet, to provide the above-stated advantages of such a system, yet overcome or substantially reduce the above-stated disadvantages thereof. The intermediate portion of the copy sheet is selectively supported and guided in a series of critically positioned baffles and guides which accommodate a speed differential between the fuser roll nip velocity and the velocity of the initial image support surface. A speed variation and differential is accommodated between the leading edge and trailing edge areas of the same final image support surface, in a manner which avoids disturbance of the unfused toner image in any area thereon.